Emergency vehicles, responding to accidents or fires, and police in pursuit often pose heightened danger to those involved at the focus of the emergency and to the population around and moving toward the scene of the emergency. Typically, the surrounding population may be completely unaware of the potential dangers approaching them or towards which they are moving, and unnecessarily place themselves at risk of injury, and potentially interfere with the emergency response. Furthermore, if they are unaware in the events outside their vehicle and are instead involved with entertainment from a local radio broadcast, they will further delay taking appropriate evasive or defensive action.
Simple tone-modulated warning transmitters which sweep the broadcast bands often sound like common man-made interference, thereby offering insufficient information to the listener to determine that there is an emergency and what action could or should be taken. Moreover, if the listener cannot determine that an emergency exist, such simple warning systems will only distract and annoy the listeners, causing them to be even less responsive to the nearby emergency. Also, the sweep or stepping of a single carrier through the broadcast band(s) favors simple alarm tone modulation as alarm voice messages would never be heard in their entirety if complete coverage of the broadcast band(s) by a single swept carrier is to be provided in reasonable time.
Full band transmissions which saturate the entire broadcast band(s) require significant amounts of carefully controlled radiated power to be effective at anyone frequency. Moreover, in the event of multiple emergency vehicles responding to the same emergency, with each emitting the same full-band alarm signal, the multiple signals will cause interference when they are in or near the same location, and thus cancel each other or cause confusion to those receiving the full band alarm signal, thereby increasing rather than reducing the danger to the nearby population.